The invention relates in general to cathode ray tube circuits, and in particular to dynamic focus signals used for focusing an electron beam on a CRT.
Cathode ray tubes (CRT) provide images on a screen by deflecting and focusing an electron beam through the use of electric and magnetic fields. When the beam leaves the cathode of a CRT, it is comparatively broad. An electrostatic focusing lens situated between the cathode and the CRT screen is adjusted such that its focal point occurs at the screen, causing the beam to converge to a tight spot as it reaches the screen. The lens adjustment thus depends on the distance the beam must travel to the screen. In most CRT's, and especially in large, relatively flat screen CRT's, the distance the beam must travel varies with the distance of deflection of the beam from screen center. Thus if a focusing lens is adjusted to provide good focus at screen center, the beam will be defocused when deflected from center.
To solve this problem, focusing lenses, driven by a focusing signal from a dynamic focus signal generator, are designed to dynamically change focal length. The dynamic focus signal generator typically generates a signal from signals proportional to beam deflection. In particular, one focusing signal typically used is proportional to the sum of the squares of the horizontal and vertical components of the beam deflection. That is, if the beam is deflected by X in the horizontal direction and Y in the vertical direction, the dynamic focus signal is proportional to X.sup.2 +Y.sup.2.
However, it has been found that the effective focal length deviates from the sum of the squares of the distance from screen center in such a way that the X.sup.2 +Y.sup.2 signal is insufficient for proper focusing of the beam at the corners of the screen as well as at the sides and the center of the screen. Interactive compromising schemes are then required whereby the corner focusing and center focusing are manually adjusted until an acceptable compromise is found for the center, corner, and side focusing.